Conventionally, a wire-wound coil having a wire wound around a core made of ferrite or the like is generally used as a coil component such as a common-mode choke coil. However, since downsizing has become an important issue in the coil component, in recent years, a common-mode choke coil of a chip type, which is manufactured by using a thin-film formation technique or a ceramic multilayer technique, has been widely used.
For example, Japanese Patent Laying-Open No. 8-203737 discloses a common-mode choke coil of the chip type. FIG. 14 of Japanese Patent Laying-Open No. 8-203737 illustrates a common-mode choke coil of the chip type in which a laminate body is formed on a first magnetic substrate by stacking an insulation layer (insulator layer) and coil patterns according to the thin-film formation technique, and thereafter, a second magnetic substrate is provided on the laminate body, and a first coil and a second coil each composed of spiral coil patterns are formed inside the laminate body.
If the common-mode choke coil is further downsized, the space for forming the coil becomes insufficient, and thus the coil has to be shortened in length, which reduces the inductance value thereof, making it difficult to ensure a high common-mode attenuation.
As a solution to the above problem, for example, as illustrated in FIG. 6 of Japanese Patent Laying-Open No. 5-291044, an approach has been considered to increase the coil length by adopting such a coil that includes a first coil pattern layer composed of a plurality of conductors, an insulator layer and a second coil pattern layer composed of a plurality of conductors. The conductors of the first coil pattern layer and the conductors of the second coil pattern layer are electrically connected alternately through the intermediary of connection members provided in the insulator layer.
According to the above approach, in order to secure the connection between each conductor of the first coil pattern layer and each conductor of the second coil pattern layer, the cross-sectional area of a connecting portion for connecting the two is needed to be increased to some extent. However, since the line width of each conductor in the coil pattern layer will increase as long as the cross-sectional area of the connecting portion is increased, which decreases the inner diameter of the coil, and thereby the inductance value cannot be ensured, which makes the common-mode attenuation become smaller.
In addition, since all of the connecting portions for connecting the plurality of conductors respectively have the same area, the connecting portions located on the outer periphery of the coil are more susceptible to stress than the connecting portions located on the inner periphery of the coil. Thus, when the common-mode choke coil is subjected to an external thermal shock repeatedly, the connecting portions located on the outer periphery of the coil may disconnect away from each other, making it difficult to ensure the connection reliability.